Sulfonic group-containing polymers are known and have been used in surfactant, emulsifier, dispersing agent, high-polymer solid electrolyte, ion-exchange membrane, etc. Meanwhile, in recent years, it has been investigated to apply the sulfonic group-containing polymers to binder resin, coating material, surface-treating agent and electrolyte for cell by utilizing the advantageous properties of the hydrophilic group (sulfonic group), i.e. dispersibility, hydrophilicity, ion-capturing property, small volume resistance (this may hereinafter be referred to as small resistance) and adhesivity to base material. In, for example, Patent Literature 1 or Patent Literature 2, it is proposed to apply a polymer (a copolymer of acrylamidesulfonic acid) to ion-exchange membrane or high-molecular electrolyte for fuel cell.
Patent Literature 1: JP-A-1999-302410
Patent Literature 2: JP-A-2002-343381
The sulfonic group-containing polymers are hydrophilic (water-soluble) owing to their hydrophilic group; therefore, when the content of hydrophilic group is higher than a certain level, the polymers are very low in water resistance and, when they have been used, for example, as a membrane material, a binder material or a coating material, the polymers swell remarkably in the presence of water, inviting problems of significant reduction in the mechanical strength of membrane, easy peeling from base material and reduction in durability. For these reasons, the content of sulfonic group has been restricted and the properties of small resistance and hydrophilicity have not been exhibited sufficiently.
Further, when there is employed a step of solvent vaporization from polymer solution, foaming appears, which becomes a defect in dried membrane.